The present invention generally relates to digital communication systems, and more particularly to multidimensional code communication systems using polarization, amplitude and phase modulation of one or a plurality of RF carriers and one or multiple pulses.
Quadrature amplitude modulation is widely used in data modems for the transmission of digital data over telephone voice channels. It is also employed in the form of four-phase PSK for PCM transmission of telephone voice channels over satellite transponders. The advantages of quadrature amplitude modulation for digital transmission have been pointed out by a number of investigators. Some have concentrated on two-dimensional signal mappings. See for example C. R. Cahn, "Combined Digital Phase and Amplitude Modulation Communications Systems", IRE Transactions on Communication Systems, December 1960, pp. 232 to 237. Other investigators have considered multidimensional codes. See for example D. Slepian, "Permutation Modulation", Proceedings of the IEEE, March 1965, pp. 228 to 236, and R. Ottoson, "Group Codes for Phase- and Amplitude-Modulated Signals on a Gaussian Channel", IEEE Transactions on Information Theory, May 1971, pp. 315 to 321.
According to one aspect of the invention, an enhanced form of quadrature amplitude modulation is obtained by introducing coding in multidimensional space. This leads to greater efficiencies in terms of power and bandwidth utilization. According to another aspect of the invention, the same multidimensional coding can be advantageously used by exploiting the two dimensions of polarization space. In either case, the multidimensional codes permit substantial reduction in bandwidth with minimum increase in power consumption. Although codes based on triangular lattices have been shown to provide the densest packing in two dimensions, practical modem implementations have generally incorporated non-optimum square rectangular grids. See M. K. Simon and J. G. Smith, "Hexagonal Multiple Phase- and Amplitude- Shift-Keyed Signal Sets", IEEE Transactions on Communication Systems, October 1973, pp. 1108-1115. G. R. Welti and J. S. Lee in "Digital Transmission with Four-Dimensional Modulation", IEEE Transactions on Information Theory, July 1974, pp. 497-502, have presented a general method for constructing compact four-dimensional codes and have calculated the performance of certain four-dimensional codes with a four-dimensional peak energy constraint. The purpose of the present invention is to provide practical modem implementations which advantageously incorporate such compact multidimensional codes.